II've seen them in midgets back in the day. Today, since water cooled motorcycle engines are more common & typically motorcycle engines are made even lighter & more compact than an outboard I don't think you'd find any advantage.
What would you do for a transmission?

Another difficulty might be that they are designed with a vertical drive shaft. Might be more difficult to transfer that to a horizontal transmission.

I'd love to see it tried though...

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Has anyone ever thought about building a bike with a outboard powerhead? 25-40HP They seem to be farly easy to find with bad lower units...and there not worth alot as boat motors.

Unless the engine is very old it is loop scavenged and will have a broad power band. The main issues will be mounting and power transfer. The problems with power transfer are basic engineering issues. Suitable transmissions can be found on older British or even Indian built Enfields and Harley's and their clones which have mostly separate transmissions. These usually have couches too. The PTO of the engine will have to be modified to accept a chain or belt drive to match the transmission. This is not hard but is not cheap for a one of. Same with engine mounts.all of this is basically engineering and fab.
As in the old commercial..if you have the time.......

Machine shop time runs about ninety bucks and hour and they have to take you cad files and make machine files which is usually a two hr job. So if you can tube jockey the design you can get it made. And it will not be cheap. And you will get a poor running fuel hog engine that is not optimized for its application..but you may have fun.

Lets face it. I made homemade bread, jam, marmalade and such. It is a hobby. Do I save money??? Only if my time and fruit are free.

But I have fun.

There is a dude that makes v twins based on Indian made Enfields. I think he sells them from 24k for a 44 hp bike. He sells a few a year. They are neat. He has fun. Australia I think.

I'm not sure if it was common, but there were some roadrace bikes in the 70's pwered by 2 stroke outboard engines. There is one here in Edmonton that shows up occasionally in the vintage section of bike shows- sorry I can't recall who built it or owns it presently, but it was either an act of ingenuity or craziness, depending on your point of view. It had a monocoque frame (not tubular), used the engine as a stressed member, and if my (admittitly dim) memory serves, an Earles type front fork. I believe it did ok. I have no idea what he/they used as a transmission. I also recall some drag bikes with outboard based engines. Several others used snowmobile engines. Ah, the 70's. There was more to it than wide bottom plaid pants and platform shoes.

Sounds like a project for that old Evinrude at the back of the shed.
Good luck!

Anything is doable if you've got the time, the power density on a typical outboard engine is not that great though, I would look towards a snowmobile or jetski engine, usually more power per cc and already have the proper orientation...likely just as or even cheaper than an outboard.

I'd thought of something like that running at torque-a-verter setup, or even an automatic atv transmission, would make a fairly interesting drag or short trip streetbike.

I know an old man who pulled the worn out engine out of his old cb350 that had been sitting for the past 40 years and bolted in a p2 caterpillar diesel trash pump engine along with an torque-a-verter from a trencher or something, said it tops out at 70mph and gets about 120mpg lol.

Anything is possible, it's all time and resources, I'd really like to build a street legal briggs powered bike, done quite a few dirtbikes with them, had a suzuki 50 that'd top out around 50 with an 8 horse briggs lol.